Air dilution attachment for explosive-gas analyzers



June 3, 1969 s. F. KAPFF 3,447,359

AIR DILUTION ATTACHMENT FOR EXYLOSI VE-GAS ANALYZERS Filed March 51,1966 Fig. I F i9. 2

5 8 v 5 20 7 Al/ I f 2 2 70 A To ,6 Gombustib/es l6 Combustib/esDetector C) Detector l6 l8 l0 l2 l0 /2 Sample 5 Sample H 4 4 O t I l lDIAL ssrrms- VALVE a 'NVENTOR Slxt Frederick Kapft United States Patent3,447,359 AIR DILUTION ATTACHMENT FOR EXPLOSIVE-GAS ANALYZERS SixtFrederick Kaptf, Homewood, Ill., assignor to Standard Oil Company,Chicago, 11]., a corporation of Indiana Filed Mar. 31, 1966, Ser. No.539,155 Int. Cl. GOln 31/00 US. CI. 73-23 5 Claims ABSTRACT OF THEDISCLOSURE This invention concerns the combination of a pneumatic bridgeand a combustible detector. The pneumatic bridge, which is provided witha pressure differential detector that indicates a pressure imbalance inthe bridge, mixes a gaseous sample and air together at a predeterminedratio. When the detector indicates an imbalanced condition, anadjustable valve in the bridge provides means for regulating flowthrough the bridge so that the mix ratio remains constant.

This invention relates to explosive-gas analyzers and more particularlyto an improved air dilution system for use with explosive-gas analyzers.

The explosive-gas analyzer or combustible detector as it is also knownis an instrument of vital importance in many chemical industries becauseof the danger of explosive-gas mixtures developing in tanks and roomsthat contain volatile chemicals. In the petroleum industry, for example,refinery units are often purged of hydrocarbon gases with an inert gassuch as nitrogen before the units are opened to the atmosphere. Thepurging must be continued until the hydrocarbon concentration is lowenough that when the vessel is opened to the atmosphere no flammablemixtures will be obtained. LID. order to follow the progress of thepurging operation, continuous measurement of hydrocarbon is needed. Theaccurate measurement of hydrocarbons in inert gas is difiicult, however,with present instruments since the usual analyzer will not function inthe absence of oxygen. Current practice has been to purge for aprescribed time which past experience or past laboratory tests has shownto produce hydrocarbon-free vessels. While such a technique is generallysatisfactory, there are two drawbacks. One is that the particular systemmay be different from its condition when the previously determined saifepurge time was determined and the other is that an operation of thistype is wasteful of both inert gas and time, since generally purging iscontinued longer than needed. If an instrument was available which gaveaccurate measurements of hydrocarbons in inert gases, thesedisadvantages could be largely eliminated.

The most commonly used detector or analyzer utilizes a typicalWheatstone-bridge circuit and is used with an open and a sealed cellforming two of the four resistances of the bridge. Each cell contains aplatinum resistance wire heated to incandescence by a battery current.By means of an aspirator the gas is caused to flow around the open cell,and a flashback arrester is used to prevent ignition of the surroundinggas by the incandescent element.

In operation the combustible gas oxidizes upon contact with the openfilament, and the heat of combustion increases the filament temperature,thereby increasing filament resistance which in turn unbalances thebridge. A galvanometer indicates this change. The galvanometer may becalibrated against explosibility in terms of the percentage of the lowerexplosive limit for each mixture of an explosive gas in air. Forhydrocarbon concentrations above the lower explosive and below the upperexplosive limit, the instrument will read elf-scale, i.e., explosive.For concentrations above the upper explosive limit readings may ice beeither on-scale or below zero. Such high hydrocarbon concentrations areeasily distinguished from readings of zero percent hydrocarbon byobserving the meter during a test with the meter initially filled withair. Because of sample dilution with the air in the meter, the readingwill first go off-scale indicating explosive. As sampling continues, theneedle will return to an on-scale reading or zero depending-upon thehydrocarbon concentration. These indications will be reversed uponclearing such a sample from the instrument with air.

Many of these instruments are standardized by balancing the resistancesof the two cells when exposed to pure air and by adjusting the supplycurrent to a fixed value by means of a variable resistance in serieswith the battery. It must be calibrated for each kind of explosive gas,or corrections obtained for gases other than that for which the originalcalibration was made.

In testing gases which are oxygen-free, it is necessary to first dilutethe sample with air and then test the resulting mixture. With accurateair dilution equipment it is possible to analyze any mixture of inertgas and hydrocarbons and determine rapidly and accurately whether aflammable mixture would result upon dilution with air. If readings, forexample, were safely below the lower explosive limit for all airconcentrations, the system or unit under consideration could be safelypurged with air.

Commercial devices are available which perform this air dilution, themost accurate being those having sample and air flows metered byrotameters and pumps. While these devices provide satisfactory data,they are far from portable and are not convenient for testing ventslocated high on a unit structure.

Another and simpler device is available which attaches to a portabledetector. It consists of a variable orifice which fixes the percentageof air mixed with the sample. Several positions are provided in order toobtain different dilution ratios. This type of dilution attachment has amajor disadvantage in field use in that the percentage of air dilutionis very sensitive to the resistance of the sample line and will changeas lines and orifices accumulate foreign matter or as different tubelengths are used. For example, it is known that for a given orificesetting, the percentage of air dilution can vary from 24% with a /3"tube 2 feet in length to 51% for the same diameter tube 4.5 rfeet inlength to 69% for a like tube 15 feet in length. Under these conditionsit is extremely difficult to obtain accurate readings.

It has now been discovered that many of these difiiculties can beeliminated or substantially reduced through the utilization of the airdilution system in accordance with this invention. The air dilutionsystem of this invention assures that a constant ratio of air to samplewill enter the detector independent of any resistance changes in theline.

Briefly, this invention comprises a pneumatic bridge system connected tothe aspirator means on the combustibles detector. The bridge has twoinlet arms, one for air and one for sample. The air inlet arm has twovalves, spaced one from the other, while the sample inlet arm has twoorifice restrictions, spaced one from the other. A pressure difierentialdetector is disposed between the two arms between the valves and theorifices for determining bridge balance. One of the valves is used tocalibrate the system for the desired percent dilution. The other valveis used to balance the bridge while aspirating air and sample throughthe system, the balance being indicated on the detector. When a balanceis reached the dilution is correct and the combustibles detector willindicate the lower explosive limit for that particular concentration ofhydrocarbon and air.

Thus a simple dilution attachment has been provided which may be usedwith a portable combustibles detector and which assures accuratedilutions despite any resistance in the sample line.

The full nature of the invention will be understood from theaccompanying drawings and the following description and claims.

FIGURE 1 schematically shows one form of the pneumatic bridge system ofthis invention.

FIGURE 2 schematically shows another form of the pneumatic bridge systemof this invention.

FIGURE 3 illustrates a calibration curve which may be utilized tocalibrate the system.

Referring now to FIGURE 1, the bridge system comprises conduit arms 2and 4. As illustrated in this figure, air enters conduit 2 and sampleenters conduit 4 by means of an aspirator bulb on the combustiblesdetector (not shown).

The air and sample enter conduit 16 and are mixed as they enter conduit18 which leads the combustibles detector.

Inlet arm 2 is equipped with valves 6 and 8. Valve 8 is used tocalibrate the instrument for a desired air dilution. A number ofpositions are provided on the valve for this purpose. Valve 6 is used tobalance the bridge system.

Inlet arm 4 is equipped with orifices 10 and 12. Each orifice has a holediameter of about 0.016". These orifices may be conveniently constructedof ordinary shim material. The inlet conduits 2 and 4 may be constructedof standard tubing.

Pressure differential detector 14 is connected across the arms of thebridge, and is adapted to indicate bridge imbalance caused by anyupstream resistance. Detector 14 is sensitive to bridge imbalance andwill operate to measure pressure differences across the bridge upstreamof valve 8 and orifice 12. Fundamentally, then, this network guaranteesthat the pressures ahead of valve 8 and orifice 12 will be equal.

In the preferred form, detector 14 is a horizontal manometer having ahorizontally disposed capillary tube containing a droplet of oil orother liquid. When the droplet is motionless, this is a very sensitiveindicator of a null balance across the bridge.

In operation air and sample are drawn through the system by operatingthe aspirator means of the combustibles detector. Usually this is in theform of a squeeze bulb. To obtain a particular dilution valve 8 is setaccording to a previous calibration. The calibration technique will beexplained with reference to FIGURE 3. Valve 6 is then adjusted untilthere is little or no motion of the oil droplet in detector 14 duringaspiration. Known dilution is thereby obtained regardless of sample lineresistance as long as measurements are taken with detector 14 indicatinga null balance or zero pressure differential across the bridge. Asindicated above, under these conditions, this system assures that aconstant ratio of air to sample will enter the combustibles detectorindependent of any resistance changes ahead of orifice 10.

While the above describes the preferred form of this invention, it is,of course, possible to practice this invention by positioning orifices10 and 12 in air inlet conduit 2 and positioning valves 6 and 8 insample inlet conduit 4. With this arrangement flow of sample would beregulated with respect to air and the same result could be achieved.

Referring now to FIGURE 2 another embodiment of the invention isillustrated. If one fixed dilution is desired, then valve 8 can be leftin one position or, as illustrated in this figure, be replaced withfixed orifice 20. As before, valve 6 is adjusted each time to zero thebridge. Other than this one change, operation of the system is identicalwith that described above. It is merely necessary to select the orificeopening which gives the desired percentage air dilution.

FIGURE 3 is an example of a calibration curve which may be used incalibrating the instrument for desired .4 air dilution. The curve 22 isa plot of percentage air dilution versus the dial setting of valve 8.For example, for a desired air dilution of 20% the dial on valve 8 isset to 940. The instrument is now calibrated to give a dilution of 20%air. If a difierent dilution is desired, it is merely necesary tocorrespondingly change the setting of valve 8 to a difierent reading,i.e., 60% dilutionvalve setting 800.

In constructing the air dilution attachment in accordance with thisinvention the Hoke Model 2RB285 needle valve may be used for valve 8 andan Ideal-Aerosmith 52-2-14 needle valve may be used for valve 6. Aspreviously pointed out, ordinary shim material may be employed in makingorifices 10, 12, and 20, and standard tubing may be utilized forconduits 2, 4, 16, and 18.

Typical combustibles detectors which are in common use and which may beemployed in conjunction with this invention are the Mine SafetyAppliance Company Explosimeter and the Davis Emergency Equipment CompanyVapotester.

This invention is able to provide a continuous range of dilutions for adetailed investigation of operating problems in systems deficient inoxygen. And in addition to its use in systems containing hydrocarbons ininert gas, this invention can also be used to dilute overrich,hydrocarbon-air mixtures to obtain on-scale readings. Through simplecalculations the original hydrocarbon concentration may be determined.For example, if 50% dilution provides a meter reading of lower explosivelimit, then the original concentration was lower explosive limit.

As mentioned previously, by simple calibration the combustiblesdetectors with which this invention may be used will respond to varioushydrocarbon-inert gas-air systems. Examples would be isobutane, ethane,propane, methane, propylene, hexane, benzene, etc.

While the foregoing specification sets forth the invention in specificterms, it is to be understood that numerous changes in the shape, size,and materials may be resorted to without departing from the spirit andscope of the invention as claimed.

Having described the invention, what is claimed is:

1. The combination comprising:

first and second conduit means each having two spaced restrictions andrespectively having air inlet means and sample inlet means, said conduitmeans merging at a point remote from said inlet means and saidrestrictions to form a common junction where air and sample are mixedtogether at a predetermined ratio;

third conduit means in advance of said common junction forinterconnecting said first and second conduit means between two pointsintermediate said restrictions;

pressure differential detector means mounted along said third conduitmeans for indicating a pressure imbalance between said first and secondconduit means; and adjustable means at one of said restrictions forcontrolling pressure in said first and second conduit means so thatpressure imbalance indicated by said detector means can be equalized tomaintain said predetermined ratio of air and sample; and

combustible detector means for detecting a combustible mixture of airand sample and having aspirating means coupled to said common junction.

2. The combination defined in claim 1 wherein another one of saidrestrictions has adjustable valve means movable to different calibratedpositions to provide different predetermined mixtures of air and sample.

3. The combination defined in claim 1 wherein the two restrictions inthe first conduit means are valves, and the two restrict-ions in thesecond conduit means are orifices.

4. The combination defined in claim 1 wherein, in

5 6 said first conduit means, one restriction is a valve and 3,334,5138/1967 Thomas 73-23 the other restrictioqis a fixed 3,354,052 11/1967Williams 73 25 5. The combination defined 1n claim 1 Wherem the3,362,228 1/1968 Stuben pressure differential detector means is ahorizontal manometer having a horizontally disposed capillary tube 5containing a liquid droplet. JAMES J. GILL, Primary Examiner.

References Cited C. IRVIN MCCLELLAND, Assistant Examiner.

UNITED STATES PATENTS 2,790,320 4/1957 Salko et a1. 73-196 10

